1. Field of the Invention
This invention relates to ultra high vacuum systems, those operating in a pressure range below 10.sup.-6 torr, and specifically provides remotely actuatable removable seals for joining concentric tubes in such systems.
2. Description of the Prior Art
In assembling ultra high vacuum (UHV) systems, those systems operating at pressures below approximately 10.sup.-6 torr, seals and closures have presented challenging problems. For such systems, rubbers and elastomers are not suitable as seals as they are permeable to gases to an extent such that very low base pressures cannot be attained and such seals typically cannot survive the high temperatures often necessary to bake out vacuum systems. When frequent separation of a joint is not a requirement, multipass tungsten-inert-gas (TIG) or metal-inert-gas (MIG) welding is often preferred to a demountable closure. When frequent separation or proximity of heat sensitive materials makes welding impractical, plastically deformed metal seals are satisfactory. The seals, however, require very rigid flanges and many closely spaced bolts to accomplish a sufficiently tight seal, and assembly and disassembly are time consuming, especially if these operations must be performed remotely. Nevertheless, plastically deformed metal seals such as gold wire and flat copper washers are the most reliable seals and are used almost universally in UHV work. Many styles have been developed, from laboratory fabricated special seals to commercially available standardized seals. The successful UHV seals all have two characteristics in common; (1) at least one of the sealing surfaces is plastically deformed, and (2) there is no relative motion between the sealing surfaces.
It is desirable to provide a seal which possesses, in addition to these two characteristics, the ability for remote actuation, installation and removal.
Additionally, where it is necessary to demountably seal large cylindrical components in a UHV environment, for example, those having diameters of several feet or more, the operational concerns are magnified, particularly where remote assembly and disassembly are required. Separating forces between the seal and the UHV system components can be large as a result of the weight of the seal members, and can be beyond the capability of force-sensitive remote actuators or manipulators. The separating forces may, of necessity, be applied through complex structures such as piping, vacuum shells and expansion joints, which are made more costly, more complicted, and less reliable by this additional function imposed upon them. Further, the sealing surfaces may be severely damaged locally by sliding them while loaded by the dead weight of the bulky seal members. Resulting repair or replacement of the seal and component surfaces can be difficult, slow and costly because of the large size of the components involved.
In metallurgy there exists high strength metal alloys which exhibit unique "memory" characteristics when undergoing a transformation between martensitic and austenitic states. The alloys can, for example, be machined in a high temperature austenitic geometry, cooled through a phase transformation to a weaker martensitic state, deformed and, upon heating through the same phase transformation temperature range, return substantially to the original austenitic geometry. If restrained from returning to the original geometry the alloys develop internal stresses consistent with the strain between the original "remembered" geometry and the restrained configuration imposed upon the alloys.
Among the alloys exhibiting these characteristics are various combinations of gold and cadmium, copper and zinc, indium and thallium, copper and tin, as well as nickel and titanium, among others, the latter being commonly referred to as Nitinol which is commercially available from the Raychem Corporation. The nickel-titanium alloys also can incorporate smaller amounts of other elements. Alloys generally exhibiting memory characteristics are also referred to as "heat recoverable" since they require heat to effect the crystallographic phase transformation necessary to recover their shape. Additional detail regarding such nickel-titanium alloys is provided in the above-referenced Buehler patent.
Also available from the Raychem Corporation are Cryocon.RTM. electrical connectors and Cryofit.RTM. piping connectors, utilizing Nitinol alloys. The Cryofit connector is essentially a sleeve of Nitinol, having internal seal ridges in series, which is bored in the austenitic phase to a diameter less than the outside diameter of pipes to be joined, then chilled and transformed to martensite, and mechanically expanded to a diameter greater than the outside diameter of pipes to be joined. The connector can be slipped over the pipes, heated and transformed to austenite, whereby a hoop stress presses the series of seal ridges into the outer surfaces of the pipes making an excellent pipe and hydraulic connection. The ridges, however, while forming a seal, are inconsistent with UHV applications, and may locally deform the joined pipes to a degree inconsistent with reuse of the connector for substantial sealing. Seal surfaces in series also raise concerns regarding virtual leaks.
It is accordingly desirable to provide a sealing arrangement for application to large UHV system components which does not require mechanical actuators and which can be remotely installed, actuated, released and returned approximately to its pre-installation configuration having sufficient clearance to facilitate removal and replacement without damage to the seal or the system components.